1. Field of the Invention
The present invention relates to a semiconductor memory, more particularly to a video random access memory (Video RAM) by a large scale integrated circuit (LSI) used as the image memory.
The Video RAM according to the present invention is advantageously used for high speed serial read/write data transfer in an information processing system.
2. Description of the Related Art
Recently, the display apparatus, particularly a graphic display apparatus, has been considerably developed in the field of information processing systems. The Video RAM used in this graphic display apparatus is generally constituted by a plurality of RAM (for example, dynamic RAM's) and shift register sets. The RAM is used for storing the image data and is randomly accessed by a memory controller. The shift register is serially accessed and used for reading/writing the data from/to the RAM. Accordingly, one of the functions required of the Video RAM is a high speed read/write operation. Another requirement is to miniaturize a structure thereof, and thus increase the packaging density of memory cell array. The Video RAM has many external pins (terminals), for example, 24 pins, in order to connect with another device, but the number of terminal that can be provided is limited due to the above-mentioned miniaturization and package densifying. Therefore, many functions are required for each terminal, for example, as a data or signal input/output terminal. In general, many input terminals and output terminals are used only for one purpose in the Video RAM. However, if the input terminal can be used in common with the output terminal, the number of pins having the input/output function can be reduced by half in the Video RAM. Accordingly, the remaining terminals can be used for other purposes, so that the Video RAM can have many other functions necessary to the graphic display.
Three relevant conventional Video RAM's are disclosed in the NIKKEI ELECTRONICS, 1985, May 20 and Aug. 12, and IEEE, International Solid-State Circuit Conference (ISSCC 85), Digest of Technical Papers, Feb. 13, 1985 pages 30 and 39.